1. Field of the Invention
This invention pertains to the manufacture of metal castings, and more particularly, it relates to heat treatment of metal workpieces.
2. Description of the Prior Art
In the manufacture of metal workpieces, such as castings of steel, aluminum based alloys and white cast iron, the workpiece is subjected to various temperature and time defined changes so that certain desired internal structural changes will occur to improve the product. These changes can generally be grouped under heat treatment.
The heat treatment procedure usually involves a two step process wherein the workpiece is subject to a first temperature over a first time period and a second temperature over a second time period, and the rate of temperature changes between the first and second temperatures can also become important to effect the desired internal structural change in the workpiece.
For example, a steel casting is rapidly cooled to complete the internal structural transformation so that the product has a certain hardness, ductility, etc. Aluminum based alloys are heated to a first temperature, slowly cooled to a second temperature whereby the alloy constituents are dissolved and then, the hardener components precipitate from the alloy.
In the manufacture of malleable iron castings, the conventional practice is to melt a charge of pig iron and steel scrap, and then pour the charge into a suitable mold to produce a hard brittle white iron casting.
Although a very limited use can be made of white iron castings, the ductility of these products is essentially zero and mechanical/thermal stresses will result in material failures especially at sharp corners. Malleable iron is produced by annealing or graphitization of the white iron castings.
Malleable iron castings find commercial uses where strength, ductility, machinability and resistance to mechanical/thermal shock are important factors. The white iron castings have been made malleable by being placed into furnaces, and their temperature raised slowly to 1500.degree. F.-1750.degree. F., held there for a period of time, and then slowly cooled at a controlled rate. This procedure may extend for 50 or 100 hours even though the actual time required for the annealing or graphitization process is less than 15 hours so that the desired conversion occurs of the combined carbon to the "temper" carbon or graphite without cracking or distorting the castings. Naturally, the furnaces must have a special atmosphere (slightly reducing) to prevent corrosion and scale or the parts must be sealed in protective material.
The furnaces can be stationary batch-type or car-bottom type, and in some cases, the castings are placed into a static bedding (gravel) to minimize distortion or warping and cracking. Even with these precautions, castings with sections over 2 inches in thickness, or that weigh over 100 pounds, do not lend themselves presently to conversion into malleable iron castings except by experience and tedious furnace operations. The basic problem appears to reside in heating and cooling the casting throughout to a uniform temperature, and to maintain within a few degrees of temperatures in the critical range so as to convert the combined carbon of the white iron casting into graphite or "temper" carbon, and thus, to produce the desired ductile property in the casting.
One problem associated with the heat treatment of a metal workpiece at elevated temperatures is that external shape changes, such as working, occur simultaneously with the desired internal structural changes. It is one purpose of this invention to control and reduce the occurance of these external shape changes during the heat treatment of a metal workpiece.